Patent Document 1 discloses a grommet that is to be attached to an attachment hole formed in a panel, the grommet including a cylinder portion provided with a through hole through which a wire harness is to be inserted, and a diameter expansion portion that is formed so as to extend outward from the cylinder portion, the outer circumferential portion of the diameter expansion portion being provided with a ring-shaped groove into which a circumferential edge portion of the attachment hole can be fit.